1. Field of the Invention
The present invention relates to an image forming apparatus, which connects an image processing means for generating pixel data and an image forming means for forming an image on a recording medium via a communication means, and a method used therefor.
2. Related Background Art
An image forming apparatus, which is constituted by a video controller for converting code data described in a printer command system such as a page description language into pixel data, and a printer engine for forming an image on the basis of the pixel data supplied from the video controller and transferring and fixing the formed image on, e.g., a recording paper sheet, is known.
A video interface connecting the video controller and the printer engine will be described below. FIG. 11 is an explanatory view showing a list of signals of the video interface. In FIG. 11, "output" represents an output from an engine controller in the printer engine to the video controller, and "input" represents an input from the video controller to the engine controller.
A signal /BD is a horizontal synchronization signal, in the main scanning direction, of the printer engine. The video controller outputs video data /VDO for one main scanning line in synchronism with the trailing edge of the signal /BD. A signal /PPRDY indicates that a power supply is connected to the printer engine, initialization of the engine controller is completed, and it is ready to communicate command status signals between the engine controller and the video controller.
A signal /RDY indicates that the printer engine is ready to perform a print operation in response to a print start command (signal /PRNT is true). The signal /RDY changes to true under conditions that the temperature of a heating roller in a fixing unit is proper (i.e., a sufficiently high temperature for fixing a toner image on a paper sheet), a recording paper sheet has not jammed, a polygonal mirror is normally rotated at a prescribed rotational speed, the signal /BD is normally output at a prescribed period, and so on.
A signal /TOP is a print synchronization signal, in the sub-scanning direction, in the printer engine. The video controller sets the image write start position in the sub-scanning direction in synchronism with the trailing edge of the signal /TOP. The position of a paper feed sensor is determined so that the time required from when the leading end of a recording paper sheet is detected by the paper feed sensor until the recording paper sheet reaches a transfer roller equals the time required until a latent image formed on a photosensitive body by a laser beam reaches the transfer roller upon rotation of the photosensitive drum. For this reason, when the engine controller detects that a paper feed sensor signal (PFSNS) from the paper feed sensor changes to true, it immediately sets the signal /TOP to be true for one sec.
A signal /SBSY is a level signal which indicates that the printer engine is transmitting a status signal (the condition of the printer) to the video controller using a signal line /SC. At this time, as the signal /SC, serial information in units of 8 bits is transmitted to the engine controller, and this signal is called a status signal.
A signal /PPOUT notifies the video controller of the fact that a recording paper sheet on which the print operation has been completed is ejected.
A signal /CBSY is a signal which indicates that the video controller is transmitting a command (a command for the engine controller) using the signal line /SC. At this time, as the signal /SC, serial information in units of 8 bits is transmitted from the video controller to the engine controller. A signal /SCLK is a synchronization clock used upon serial transmission of the signal /SC (status and command).
A signal /PRNT is a signal with which the video controller instructs the printer engine to start the print operation. In a continuous print mode, this signal means continuation of the print operation.
A signal /CRPDY indicates that a power supply voltage is supplied to the video controller, the initialization of the video controller is completed, and the video controller is ready to communicate commands and status signals with the printer controller.
A signal /VDO is an image signal to be printed by the printer engine. This image signal is output by converting image code data received by the video controller from a host computer into dot data.
The formats of commands sent from the video controller to the engine controller and the status signals sent from the engine controller to the video controller exchanged via serial communications will be explained below.
As described above, command information and status information are 8-bit serial signals. When 1-byte command information is supplied from the video controller to the engine controller in synchronism with the signal /SCLK, the engine controller sends back 1-byte status information to the video controller. This command information includes two commands, i.e., a status request command for checking the status of the printer engine, and an execution command for instructing the printer to perform some operation.
Each of the command information and status information has the eighth bit as a parity bit, and an odd parity is added upon transmission. The status information sent back by the engine controller has the first bit as a command error bit. That is, when a command supplied from the video controller is not predetermined data, or a parity error has occurred, the engine controller sets "1" in the first bit of status information, and sends back the status information.
FIG. 12 is an explanatory view showing an example of status information when no command error has occurred, i.e., the first bit is "0". FIG. 13 is an explanatory view showing status information when an error has occurred. When an error has occurred, "1" is set in the first bit, and the type of the error is specified by the second to seventh bits.
Basic status information when no error has occurred is sent back when the video controller sends "01" as a status request command, and six different types of status information except for the command error bit (first bit) and the parity bit (eighth bit) are included in this information.
More specifically, when "1" is set in the second bit, it indicates that the printer engine is ready to receive a signal PRNT. When "1" is set in the third bit, it indicates that the printer engine is transporting a recording paper sheet. When "1" is set in the fourth bit, it indicates that a recording paper sheet jam or a print error has occurred in the printer engine, and print data must be re-transmitted to the engine controller.
When "1" is set in the fifth bit, it indicates that the printer engine is waiting since the temperature of the heating roller in the fixing unit has not reached an appropriate temperature. When "1" is set in the sixth bit, it indicates that the printer engine is pausing due to a pause command from the video controller.
When "1" is set in the seventh bit, it indicates that the printer engine has failed and must be corrected by calling a service person or operator.
In addition, the engine controller has status signals for reporting, to the video controller, a recording paper sheet jam, the absence of paper sheets, the open state of the front door, a fixing failure, a motor failure and the like in response to the command from the video controller.
However, in such apparatus, since the video controller supervises the conditions of the printer engine using serial communications, the following problem is posed.
That is, even when the condition of the printer engine remains the same, the video controller must perform serial communications to supervise a plurality of status bits, and the communication processing exerts a heavy load on the video controller.